(1) Present Status of Malaria and Other Infections
In spite of the optimistic prospective of overcoming various infections, the global transportation of people and products, the development-inducing global environmental change, and the drastic change of various social activities have significantly and rather adversely changed the circumstances surrounding the infections. For example, the popularity of overseas travels and the destruction of rainforests induce tropical diseases, and the heavy use and the abuse of anti-infectives lead to the appearance of drug-resistant viruses and bacteria.
Among the variety of infections, especially the actions against malaria as a tropical protozoal infection are slow and insufficient. The number of malaria patients amounts to approximately 300 to 500 millions per year including 1.5 to 2.7 million deaths (see WHO report, 1999). Among four human-infecting protozoa species, Plasmodium falciparum is most severe and deadly. Malaria has the massive impact on the human health and also causes the economic recession and social instability in African states.
It is often pointed out that the destruction of rainforests and the global warming have contribution to the increasing number of malaria patients. According to the reports of the International Panel on Climate Change (1996 and 1998), a potential increase of 50 to 80 million new malaria patients would be predicted by a temperature rise of 2° C. by the global warming. With the wide spread of overseas travels, the number of malaria-(imported malaria-) infected individuals in Japan has an increasing tendency to 120 to 150 patients per year from 50 to 70 patients per year in 1980s.
(2) Prior Arts and Their Problems
Many of currently used malaria medications include the patent loyalties and are thus rather expensive. This interferes with the widespread of the malaria medication in the developing countries. There are some inexpensive malaria drugs like chloroquine. The limitless use of these popular drugs leads to the high degree of drug resistance.
There are many malaria-related issues to be solved. The domestic and international pharmaceutical industries have not actively been involved in development of therapeutic and preventive medicines for malaria. The target of their research and development is focused on the age-related disorders and diseases, which have the greater importance for the developed countries. Another reason for their sluggish attitude is a relatively small market of the products for the developing countries. The existing companies may thus not be sufficiently reliable for development of novel antimalarial pharmaceutical substances. There is a need of commercially production and distribution of effective but inexpensive therapeutic, preventive, and inspective medicines for malaria.
The compound “mefloquine” developed during the Vietnam War is the most commonly used antimalarial drug at present. The newer antimalarial drug is Malarone approved in the US in 2000. This is, however, only the diversion of a known substance to the therapeutic medicine for malaria. The latest study has proposed an inexpensive synthetic compound OZ227 having the similar action mechanism to that of a known natural therapeutic agent ‘artemisinin’ (see Non-Patent Reference 1).
The conventional anti-Plasmodium falciparum drugs, however, generally have severe side effects including headache and nausea. Administration of such medicines for the preventive purpose is thus not generally recommendable. Some of the conventional antimalarial substances, for example, quinine and chloroquine, are deleterious. Peptide vaccines mainly composed of amino acids, on the other hand, exert only desired preventive immunological effects, while being less poisonous and deleterious than the conventional antimalarial substances.
The disadvantage of the peptide vaccine is the potentially different preventive immunological effects among different individuals. The prevalence rate of malaria is high in the epidemic regions. The simple reduction in the risk of malaria development is thus expected to have sufficient contribution to the decrease in the number of sufferers and the number of deaths.
Over many years, the inventors of the present invention have been occupied in development of peptide vaccines, based on the epidemiologic study in the epidemic regions in combination with the molecular analyses in the laboratory scale. As the result of the extensive studies, the inventors have found that enolase, which is a glycolytic enzyme produced from the Plasmodium falciparum-infected human, functions as a protective immune molecule against malaria and have developed peptide vaccines by taking advantage of such finding.
For example, the inventors noted and examined a partial amino acid sequence of enolase from Plasmodium falciparum shown in SEQ ID NO: 12.
A small amount (several milligram level) of a peptide including this partial amino acid sequence (SEQ ID NO: 12) was synthesized and was used as an artificial antigen. The use of this artificial antigen induced an immunological response to Plasmodium falciparum, enabled diagnosis of the immunological state of malaria infection (immunological inspection), and derivatively produced an immunological antigen for inhibiting the proliferation of Plasmodium falciparum. These research results are reported in Patent Reference 1, together with the research results on synthetic peptides comprising other partial amino acid sequences of enolase protein from Plasmodium falciparum. 
The peptide vaccine is conventionally produced by the solid-phase synthesis or by the genetic recombination. Neither the solid-phase synthesis technique nor the genetic recombination technique requires the specific skill or experience for producing any sequences. The solid-phase synthesis technique, however, generally has a lower synthesis yield for a longer chain substance and gives a relatively poor yield of a final product after purification. The genetic recombination technique also generally gives a poor yield of a final product.
The laboratory scale synthesis of the peptide including the partial amino acid sequence of enolase from Plasmodium falciparum (SEQ ID NO: 12) by the solid-phase synthesis technique gives a yield of only several hundred micrograms to several milligrams. The synthesis of the peptide by the genetic recombination technique with culture of Eschelichia coli in a relatively large (laboratory scale) 2-liter vessel gives a yield of only 1 to 2 milligrams (as the vaccine for only one person). Even the genetic recombination with an industrially largest-level 1000-liter vessel gives a yield of only 500 to 1000 milligrams (as the vaccine for about 500 people).
Another conventionally known technique potentially applicable for synthesis of the partial peptide (SEQ ID NO: 12) is fragment condensation. In this case, synthesis of a fragment having a glutamic acid residue at the terminal is the most important issue to be solved. Introduction of trichloroethyl ester group or another optional ester group into N-α-protected-L-glutamic-γ-benzyl ester is essential for obtaining a synthesis intermediate N-α-t-butoxycarbonyl-L-glutamic-γ-benzyl ester-α-trichloroethyl ester described later in Examples and other optional synthesis intermediates N-α-protected-L-glutamic-γ-benzyl ester-α-protected esters. According to Non-Patent Reference 2, esterification of N-α-protected-L-glutamic-γ-benzyl ester for the purpose of protecting the α-site carboxylic acid group has a high potential for racemization. It is thus believed in the art that N-α-protected-L-glutamic-γ-benzyl ester is not suitable for the synthesis of a peptide. There has been no synthesis tried after the report of the Non-Patent Reference 2.
The synthesis of the partial peptide (SEQ ID NO: 12) by the conventional fragment condensation technique in consideration of the potential racemization requires large fragments having at least 14 residues. The desired size of each fragment is generally 5 to 7 residues at the maximum for the good yields of synthesis and purification. Namely the conventional fragment condensation technique is not adequate for the efficient large-scale synthesis of the peptide.
As used herein: Patent Reference 1: Japanese Patent Laid-Open Gazette No. 2002-371098; Non-Patent Reference 1: pages 900-903, vol. 430, 2004, Nature; and Non-Patent Reference 2: pages 1962-1965, vol. 47, 1982, Journal of Organic Chemistry.